1. Field of the Invention
The present invention relates to radar motion sensing, and more particularly to pulse-Doppler radar motion sensors.
2. Description of Related Art
CW Doppler radar motion sensors emit a continuous wave carrier and mix the transmitted RF with the return echoes to produce a difference frequency equal to the Doppler shift produced by a moving target. CW Doppler sensors have a number of serious deficiencies that limit their widespread application: 1) lack of a definite range limit, which leads to false triggers on distant clutter 2) extremely high sensitivity at close range, causing false triggering on nearby insects or vibrating objects, 3) high current consumption due to CW operation, making battery operation impractical, and 4) inability to collocate non-channelized sensors due to mutual interference.
A pulse Doppler motion sensor was described in U.S. Pat. No. 4,197,537 to Follen et al. and similar devices are described elsewhere. A short pulse is transmitted and its echo is self-mixed with the transmitted pulse so the pulse width defines the range-gated region. When the transmit pulse ends, mixing ends and target returns arriving after the end of the transmit pulse don""t get mixed and are thereby gated out. With 100% amplitude modulation, response beyond the maximum range is zero-there is no leakage. While pulse Doppler exhibits. excellent range gating characteristics, its voltage response versus range varies with 1/R2, where R=range to target. This 1/R2 characteristic occurs with CW Doppler as well (see FIGS. 3A, 3B and 7A). Thus, a large target at 10 meters range and an insect a 1 cm range may produce the same response.
A Differential pulse Doppler motion sensor disclosed in U.S. Pat. No. 5,966,090, xe2x80x9cDifferential Pulse Radar Motion Sensor,xe2x80x9d to McEwan, alternately transmits two pulse widths and subtracts the Doppler responses from each width to produce a range gated Doppler sensing region having a fairly constant response versus range (see FIGS. 3C and 7B). Unfortunately, the response at very close range, while much improved over CW Doppler, increases more than can be tolerated for some applications, such as an outdoor security alarm where insects, birds, or hail may falsely trigger the sensor.
Impulse radar, such as that described in U.S. Pat. No. 5,361,070, xe2x80x9cUltra-Wideband Radar Motion Sensor,xe2x80x9d to McEwan produces a very narrow sensing region that is related to the transmitted impulse width (see FIG. 3D). The narrow sensing region defines a thin spherical xe2x80x9cbubblexe2x80x9d about the radar. Unfortunately, most motion sensor applications require a xe2x80x9cvolume fillxe2x80x9d, i.e., a filled zone or range span, and not just a thin shell that might be pierced without detection by a fast-moving burglar. Impulse radars also suffer from the same flaw seen in all prior radar motion sensors: the gate or mixer pulse radiates and can be reflected back by a close-in target to create an excessively high response to close-in objects. This problem might be called xe2x80x9cclose-in homodyning.xe2x80x9d An impulse radar operates with very short pulses, so one might expect the close-in homodyning region to be very small, perhaps having a radius of only 1 cm. However, antenna ringing in practical antennas can stretch the close-in homodyning region to several feet. Impulse radar operation is currently prohibited in the U.S. and globally.
A two-pulse Doppler radar motion sensor, as first described in U.S. Pat. No. 5,682,164, xe2x80x9cPulse Homodyne Field Disturbance Sensor,xe2x80x9d to McEwan, transmits a first pulse and after a delay generates a second pulse that mixes with echoes from the first pulse. Thus a range gated sensing band, or thick shell, is formed with defined minimum and maximum ranges (see FIG. 3E). This is a nearly ideal motion sensing response, since both the minimum and maximum range are readily adjustable and a volume fill is provided. Unfortunately, there is a close-in homodyning response due to self-mixing of radiated and reflected components of the individual first and the second pulses.
Clearly, there is a need for improvement if radar motion sensors are to find their way into widespread use where close-in false targets are common. The modulated pulse Doppler motion sensor of the present invention resolves these prior limitations.
The present invention generates repeated sets of two pulses: a transmit pulse and a mixer pulse, with a modulated interval between the two pulses. (As used herein in the context of RF circuits and techniques, a xe2x80x9cpulsexe2x80x9d is typically a burst of multiple RF cycles.) The mixer pulse is timed to coincide with echo pulses reflected from a desired range gated region. The timing of the mixer pulse relative to the transmitted pulse is modulated, while the timing of the echo pulse is fixed by the target range, so the phase of the echo relative to the mixer pulse is modulated, which induces amplitude modulation on the mixer output. Thus, the echo pulse is amplitude modulated at the mixer output while mixer outputs from the transmitted and mixer pulses resulting from self-mixing are not modulated. The modulated output is filtered from the unmodulated components so the radar detects only the distant echo returns and filters out the close-in returns that result from self-mixing. There is no sensitivity at close-in range-a very desirable feature, and unlike the prior art.
The apparatus includes an RF oscillator with associated pulse generating and delay elements to produce the transmit and mixer pulses, a single transmit (TX)/receive (RX) antenna or a pair of separate TX and RX antennas, and an RF receiver, including a detector/mixer with associated filtering, amplifying and demodulating elements to produce a range gated Doppler signal from the mixer and echo pulses.
A further aspect of the invention includes a quadrature RF receiver and single sideband (SSB) Doppler processing. Accordingly, an upper sideband channel (USB) responds to inbound objects, while a lower sideband channel (LSB) responds to outbound objects. Doppler signals in each channel are rectified and averaged prior to threshold detection. The amount of averaging determines how far an object must move before detection. This can be termed one-way xe2x80x9cdisplacementxe2x80x9d sensing, rather than motion sensing. Displacement sensing is far more robust than hair-trigger Doppler motion sensing.
A threshold detector is connected to the upper sideband channel for inbound displacement sensing. Its threshold is set in part by signals from the opposite, or outbound channel, which forms a constant false alarm rate (CFAR) configuration. Any noise appearing in both channels will increase the CFAR detector threshold. Common channel noise may result from vibrating objects, buzzing insects, fluttering leaves, or RF interferencexe2x80x94in short, all the usual false trigger sources. With the CFAR configuration, only an object that has moved through a predefined distance in one direction, usually inbound, will trigger the CFAR detector and subsequent processor/alarm.
A primary object of the present invention is to provide a sensor with a Doppler response within a range-limited region and no response outside that region.
Another object of the present invention is to provide a spread-spectrum microwave motion sensor that can be collocated with other spectrum users without having to set a specific operating frequency.
Yet another object of the invention is to provide a motion sensor with reduced power consumption, high rejection of power supply variations and low 1/F noise in the receiver.
A further object of the invention is to provide a motion sensor with the above-cited features and direction sensing capability, and further incorporating a novel CFAR detector.
Another object of the invention is to provide a motion sensor with the above-cited features and multiple range-cell operation.
Yet another object of the invention is to provide a motion sensor with improved clutter rejection for near surfaces such as the chest wall, for cardiac motion sensing.
The present invention is a cost-effective, low power, and long lasting electronic sensor that is impervious to harsh environmental conditions such as dirt, rain, snow, acoustic noise, external thermal effects, and sunlight. Furthermore, the sensor of the present invention may use frequencies that can penetrate certain materials (without damaging the material) to allow installation behind plastic panels or wood or concrete walls.
Uses for the present invention include indoor and outdoor security alarms, home automation and lighting control, industrial and robotic controls, automatic toilet and faucet control, automatic door openers, vehicle backup warning and collision detection, and general appliance control.
The Doppler passband of the present invention can be set to pass audio and higher frequencies making it responsive to vibrations. As a vibration sensor, the present invention can be used for industrial applications such as wheel and fan blade balancing. It can also be used for shaft vibration sensing, loudspeaker sensing and control, guitar string and musical instrument pickup, and vocal cord vibration sensing. Other applications include RF identification and innumerable other applications involving active reflectors, often employing SSB radar operation, as detailed in U.S. patent application Ser. No. 09/388.785, xe2x80x9cSSB Pulse Doppler Sensor and Active Reflector System,xe2x80x9d by McEwan.
In another embodiment of the present invention, body organ motion can be detected and monitored, including cardiac motion, arterial pulse, vocal cord and tongue motion.